How much mercury is in your sushi?

A new study combining DNA barcoding and mercury analysis finds that, mercury content in tuna sushi told in supermarkets and restaurants varies by species, and that, in some cases, exceeds recommended amounts. The study, published online today in the journal Biology Letters was based on 100 samples of both akami (lean red tuna) and toro (fatty tuna) taken from 54 restaurants and 15 supermarkets in Colorado, New Jersey and New York, according to the AFP. Researchers from the American Museum of Natural History used DNA testing to determine the species of tuna, which was tested for mercury at Rutgers University. The U.S. Environmental Protection Agency recommends that people consume no more than .1 micrograms of mercury per kilo (2.2 lbs.) of body weight per day, yet the average concentration of mercury in all of the sushi samples exceeded this amount, according to the study.

As the AFP explains:

Calculated on the basis of a 60-kilo (132-pound) adult woman consuming a single order, samples of Bigeye tuna toro were found to have average mercury levels of 0.351 microgrammes per kilo, while Bigeye tuna akami had 0.344 microgrammes… Bluefin toro samples had the equivalent of 0.123 microgrammes per kilo of bodyweight per day, and bluefin akami 0.180… Yellowfin tuna, found in the samples only as akami, had 0.164 microgrammes of mercury per kilo of bodyweight.

The study found that sushi sold in supermarkets tended to have lower concentrations of that sold in restaurants, and that Yellowtail (Yellofin tuna) had less mercury than other tuna species. As a reporter at Washington, D.C.-based WTOP explains:

One reason yellowtail has lower mercury levels is the species is usually harvested at a younger age… Bigger tuna like bluefin and bigeye are warm-blooded. They need to eat more to keep up their energy, so the level of toxins in their systems tend to build up over time, the study says.

The study authors point out that there are currently no requirements for restaurants and supermarkets to clarify which species of tuna they are selling, but that clearer labels might enable consumers to exercise more control over what they eat—and in turn, how much mercury they consume.